This invention relates to relatively short peptides about 14-17 residues in length, which are naturally available in minute amounts in the venom of the cone snails or analogs to the naturally available peptides, and which include two cyclizing disulfide linkages.
The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference, and for convenience are referenced in the following text by author and date and are listed alphabetically by author in the appended bibliography.
Mollusks of the genus Conus produce a venom that enables them to carry out their unique predatory lifestyle. Prey are immobilized by the venom that is injected by means of a highly specialized venom apparatus, a disposable hollow tooth that functions both in the manner of a harpoon and a hypodermic needle.
Few interactions between organisms are more striking than those between a venomous animal and its envenomated victim. Venom may be used as a primary weapon to capture prey or as a defense mechanism. Many of these venoms contain molecules directed to receptors and ion channels of neuromuscular systems.
The predatory cone snails (Conus) have developed a unique biological strategy. Their venom contains relatively small peptides that arc targeted to various neuromuscular receptors and may be equivalent in their pharmacological diversity to the alkaloids of plants or secondary metabolites of microorganisms. Many of these peptides are among the smallest nucleic acid-encoded translation products having defined conformations, and as such, they are somewhat unusual. Peptides in this size range normally equilibrate among many conformations. Proteins having a fixed conformation are generally much larger.
The cone snails that produce these toxic peptides, which are generally referred to as conotoxins or conotoxin peptides, are a large genus of venomous gastropods comprising approximately 500 species. All cone snail species are predators that inject venom to capture prey, and the spectrum of animals that the genus as a whole can envenomate is broad. A wide variety of hunting strategies are used, however, every Conus species uses fundamentally the same basic pattern of envenomation.
Several peptides isolated from Conus venoms have been characterized. These include the .alpha.-, .beta.- and .omega.-conotoxins which target nicotinic acetylcholine receptors, muscle sodium channels, and neuronal calcium channels, respectively (Olivera et al., 1985). Conopressins, which are vasopressin analogs, have also been identified (Cruz et al. 1987). In addition, peptides named conantokins have been isolated from Conus geographus and Conus tulipa (Mena et al., 1990; Haack et al., 1990). These peptides have unusual age-dependent physiological effects: they induce a sleep-like state in mice younger than two weeks and hyperactive behavior in mice older than 3 weeks (Haack et al., 1990). Recently, peptides named contryphans containing D-tryptophan residues have been isolated from Conus radiatus (U.S. Ser. No. 60/030,722), and bromo-tryptophan conopeptides have been isolated from Conus imperialis and Conus radiatus (U.S. Ser. No. 08/785,534).
Neuronal nicotinic acetylcholine receptors (nAChRs) are believed to be heteropentameric ion channel complexes generally requiring at least two different subunits (an .alpha. and a .beta.). Molecular data indicate that in the mammalian central nervous system there exists a large number of different nAChR subunits. To date, seven different .alpha. subunits (.alpha.2-.alpha.7, .alpha.9) and three different .beta. subunits (.beta.2-.beta.4) have been defined by cloning. The .alpha.3.beta.4 nAChR subtype and the .alpha.3.beta.2 nAChR subtype are each present in the autonomic nervous system and in the central nervous system. The .alpha.7 nAChR subtype is also present in the autonomic nervous system.
While postsynaptic nAChRs have been recognized for some time, more recent data have demonstrated the presence of presynaptic neuronal nAChRs. Agonist stimulation of presynaptic nAChRs induces neurotransmitter release. Nicotinic agonists have been shown to elicit the release of several different neurotransmitters, including dopamine from striatum and frontal cortex (El-Bizri and Clarke, 1994; Grady et al., 1992; Rapier et al., 1988); norepinephrine from hippocampus (Clarke and Reuben, 1996; Rowell and Winkler, 1984; Sacaan et al., 1995; Wilkie et al., 1993); glutamate from cortex, medial habenula nucleus and hippocampus (McGehee and Role, 1995; Vidal and Changeux, 1993; Gray et al., 1996); GABA from interpeduncular nucleus (Mulle et al., 1991) and acetylcholine for cortex and hippocampus (Lapchak et al., 1989; Rowell and Winkler, 1984).
In addition, it appears that distinct subtypes of presynaptic nAChRs regulate the release of different neurotransmitters. For example, nicotine-stimulated glutamate and acetylcholine release are blocked by .alpha.-bungarotoxin suggesting that these nAChRs include an .alpha.7 subunit (McGehee and Role, 1995). In contrast, nicotine-stimulated dopamine release is not blocked by .alpha.-bungarotoxin (Grady et al., 1992). Furthermore, the nAChRs modulating norepinephrine release pharmacologically differ from those modulating the release of glutamate, acetylcholine or dopamine (Clarke and Reuben, 1996; Sacaan et al., 1995).
The possibility of selectively modulating the presynaptic release of specific neurotransmitters and the possibility of selectively targeting specific nAChRs has significant therapeutic applications. One example of a therapeutic application is tobacco addiction. Studies of nicotine self-administration in animal models suggest that block of nAChRs decreases the reinforcing properties of nicotine. Examples of therapeutic applications resulting from selectively targeting the nAChRs of the autonomic nervous system are the treatment of cardiovascular disorders, gastric mobility disorders and urinary incontinence.
It is desired to identify additional compounds which target different nAChR subtypes as well as the nAChR subtypes of the autonomic nervous system and the central nervous system. It is further desired to identify compounds which are useful as cardiovascular agents, gastric motility agents, urinary incontinence agents and anti-smoking agents.